1. Field of the Invention
This invention relates to monomers and polymers prepared therefrom. More specifically, this invention embraces electron acceptor monomers and polymers which are suitable for use in electrophotographic imaging members and reproduction systems.
2. Description of the Prior Art
In the electrophotographic arts the photoresponsive component of the imaging member has been traditionally constructed so that one layer of photoconductive material has been primarily responsible for the absorption of imaging energies, the generation of charge carriers in response thereto and the transport of such charge carriers throughout the bulk of the layer. The electronic properties of the materials used in such a layer should be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface of said layer. The failure of a material to return to its relatively insulating state prior to the succeeding charging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue", has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling imaging system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracence, other organic photoconductive materials, most notably poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography. Most organic photoconductive materials including poly(N-vinylcarbazole) lack the inherent photosensitivity to be competitive with selenium. This need for enhancement of the photoresponse characteristics of organic photoconductors thus lead to the formulation of these organic materials with other compounds, commonly referred to as "activators". Poly(vinylcarbazoles), for example, when sensitized with 2,4,7-trinitro-9-fluorenone exhibit good photoresponse and discharge characteristics and (depending upon the polarity of the surface charge), low dark decay, U.S. Pat. No. 3,484,237. Ordinarily, the bulk absorption of activating electromagnetic radiation and the consequent generation of charge carriers can and often does result in some trapping of at least one species of charge carrier within the photoconductive layer and thus some impairment in the cycling characteristics of the imaging member. This disadvantage is also present where the absorption of imaging energies and the generation of charge carriers is performed by one component of a binder layer (hereinafter functionally designated as the "charge carrier generating material") and the transport of charge carriers through the bulk of said layer by a second chemically distinct component (hereinafter referred to as "electronically active matrix material"), U.S. Pat. No. 3,121,007 and U.K. Pat. No. 1,343,671.
In order to avoid the cycling limitations often inherent in such single layered systems, it has been proposed that the functions of (a) charge carrier generation (resulting from photo-activating) and (b) charge carrier transport can be performed more satisfactorily--(with respect to cycling)--where each of these two separate functions is performed by contiguous but separate layers (U.K. Pat. No. 1,337,228 and Can. No. 932,199). In these multi-layered configurations, absorption of imaging energies and generation of charge carriers is exclusively limited to the layer of photogenerator materials. Substantial absorption and photogeneration of charge carriers within the bulk of the charge carrier transport layer can reportedly impair the cycling characteristics of this type of composite and thus is to be avoided. In U.K. Patent '228 the transport layer is capable of facile transport of either holes or electrons which are injected into it from the layer of light-absorbing charge carrier generating materials contiguous therewith. In Can. Patent '199 the charge carrier transport layer is capable of facile transport of electrons injected into it from a contiguous layer of light-absorbing charge carrier generating material. Neither patent specifically discloses a polymer having an electron acceptor moiety capable of satisfactory performance in such a transport layer. The Canadian patent does indicate that such polymers can be expected to perform in a manner equivalent to binder layers containing electron acceptor material.
Monomers having relatively weak electron acceptor groups pendant therefrom are disclosed in U.S. Pat. No. 3,418,116 and U.S. Pat. No. 3,697,264. In each distance these monomers are copolymerized with a second monomer having pendant therefrom an electron donor group. The resulting polymers reportedly are photoconductive due to the charge transfer interaction between adjacent moieties of differing electron affinites.
Attempts to prepare monomers having relatively strong electron acceptor groups (groups having an electron affinity in excess of about 0.7 electron volts) have been generally unsuccessful. This fact is borne out by the relatively few disclosures of strong electron acceptor functional monomers reported in the technical literature.
Ordinarily, the preparation of copolymers having strong electron acceptor groups appended from their backbone is beset with a number of difficulties. Due to the strong electron affinity of such pendant groups, it is virtually impossible to initiate polymerization of such monomers by free-radical techniques, since the electron acceptor moiety quenches the free radical prior to substantial polymerization of the monomer. This problem has led to attempts at introducing electron withdrawing substituents on groups pendant from a preformed polymer which does not already inherently possess strong electron acceptor properties. This technique also encounters serious synthesis hurdles since attempts at, for example, nitration of poly(vinylfluorenone) results in degradation of the polymer and reduction in its solubility in common solvents (presumably due to cross-linking).
In both U.K. '228 and Can. '199 discussed previously, it was indicated that electron acceptor systems can be prepared by dispersing and/or dissolving a nonpolymeric electron acceptor in a suitable binder and casting or coating this composition as a film on a layer of charge carrier generating materials. In terms of long term cycling stability, such binder system transport layers are not equivalent to transport layers prepared from polymers. Such binder layers can at best be described as metastable, undergoing a progressive decline in their electronic properties. Such instability is believed to be due in part to the tendency of such nonpolymeric materials to migrate within the polymeric binder and thereby cause phase separation due to crystallization. In certain instances, such nonpolymeric materials can diffuse out of the polymeric binder, thereby rendering the charge carrier transport layer electronically inert. In the event of either occurrence, such binder layer transport layers would be precluded from use in a composite photoconductive layer requiring repeated cycling of this imaging member over an extended period of time, since the electronic properties of the imaging member would not be capable of remaining within the machine specifications for such a device. The electron transport layer configuration of the multi-layered photoconductor referred to in the above patents is superior to the hole transport layer system in that the electron transport system is relatively insensitive to oxidative degradation and unlike the hole transport analog, is capable of maintaining more stable electronic performance, thus, prolonging its useful lifetime within an electrophotographic reproduction system.
Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the object of this invention to provide a process for preparation of monomers which can be readily polymerized to polymers having the capability for facile electron transport.
It is another object of this invention to provide a multi-layered photoconductive composite wherein charge carrier generation and charge carrier transport are preformed by separate but contiguous layers.
It is yet another object of this invention to employ such multi-layered photoconductive composite in an electrophotographic reproduction method.